1. Field of the Invention
The present invention generally relates to an air-fuel ratio (e.g., oxygen concentration) sensor detecting an oxygen concentration of intake or exhaust gas in an intake or exhaust passage of an internal combustion engine. More specifically, the present invention relates to a structure of an oxygen sensor using a solid electrolyte.
2. Related Art
An adjustment of an air-fuel ratio in an internal combustion engine is very important for realizing the saving of energy (i.e., saving of fuel) and purification of exhaust gas. An oxygen sensor is used to detect the air-fuel ratio. A conventional oxygen sensor comprises a solid electrolyte which conducts oxygen ions. A pair of electrodes are provided on the inside and outside surfaces of a solid electrolyte. A gas anti-diffusion layer is added. They constitute an electrochemical cell serving as an oxygen sensor element. When the internal combustion engine is in an idling or start-up condition, the temperature of the exhaust gas is low. A heater unit is disposed inside the oxygen sensor. This heater is activated to increase the temperature of the oxygen sensor element when the internal combustion engine is in such an idle or start-up condition. Thus, the output of the oxygen sensor is stabilized.
More specifically, as shown in FIG. 15, an oxygen sensor 9 comprises a cup-shaped oxygen sensor element 91 serving as an electrochemical cell. The oxygen sensor element 91 is accommodated in a container 90 (refer to Japanese Patent 8-2514000). The container 90 comprises a housing 93 covering a front end of the oxygen sensor element 91 and a base body 92 located at a base end of the housing 93. A flange 931 is formed at a center of the housing 93. The flange 931 is fixed to an appropriate portion of an exhaust gas passage pipe. The oxygen sensor element 91 is fixed to the housing 93 via a talc 932.
An element protecting cover 935 is attached at a lower end (i.e., front end) of the housing 93. The element protecting cover 935 is inserted in the exhaust passage pipe and exposed to the exhaust gas. Atmospheric cover members 921 and 922 are provided at a base end side of the base body 92.
The atmospheric cover members 921 and 922 are made of metallic thin plates. The cover members 921 and 922 are installed to an upper end of the housing 93. The cover member 921 is fixed to the upper end of the housing 93 by caulking. Then, the cover member 922 is attached to the outer surface of the housing 93. Then, the cover member 921 and the cover member 922 are fixed each other by caulking.
A plurality of gas holes 936 are opened on the element protecting cover member 935. The gas holes 936 introduce the exhaust gas inside the element protecting cover member 935.
A heater unit 96 is inserted inside the oxygen sensor element 91. The heater unit 96 is supported by the oxygen sensor element 91. The oxygen sensor element 91 comprises a cup-shaped solid electrolyte. A reference electrode 94 and a measuring electrode 95 are provided on the opposed surfaces of the solid electrolyte. The electrodes 94 and 95 of the oxygen sensor element 91 are connected to signal output leads 940 and 950 via primary leads 941 and 951 and relay leads 945 and 955, respectively. The leads 940 and 950 serve as output terminals for transmitting the detection signals of the oxygen sensor element 91 to an external device, such as an engine control unit. The heater unit 96 is connected to a lead 960. Electric power is supplied to the heater unit 96 through the lead 960.
However, as described above, the base body of the conventional oxygen sensor is basically constituted by a plurality of cover members made of metallic thin plates. The structure of the above-described conventional base body is complicated. Installation of these cover members is troublesome and time-consuming.